X-ray fluorescent screen



Sept. 14, 1954 W 1 HUSHLEY 2,689,189

X-RAY FLUORESCENT SCREEN Filed Dec. 5, 1951 WITNEssEs; lNvENToR Woller J. Hushley Patented Sept. 14, 1954 MNHTED STATES ATENT OFFICE A X-RAY FLUORESCEN'I SCREEN Application December 5, 1951, Serial No. 260,006

7 Claims.

My invention relates to fluorescent screens and in particular relates to screens in which a thin layer of fluorescent material is closely associated with a layer ot photo-emissive material to produce an electron image having a space-distribution which is a replica of the space-distribution of a radiation pattern. One use of such screens is in image intensiiiers of the type described in Mason and Coltman, U. S. Patent 2,523,132 for an X-ray image intensifier. In that arrangement X-rays transmitted through an observed object generate a light-image in a uorescent layer which is positioned in close contiguity to a photoelectric layer, and the electron-image resulting in the latter is accelerated into impact upon an electron-phosphor output screen on which it produces an image of highly intensified brightness compared with the original light-image. Thus an observer may see on the output-screen an accurate reproduction of the X-ray image of the object viewed, enhanced in brightness over a hundred fold.

The accuracy with which the electron-image corresponds to the original light-image generated in the fluorescent input screen is naturally dependent upon a close-spacing of the photoemissive layer from the iluorescent layer; but it is desirable to separate the two layers by an impervious layer of transparent material since the highly eii'icient fluorescent substance is greatlyl injured by contact with vapors, even in minute U amounts, of the most effective photo-emissive materials. A practical problem therefore arises in constructing a screen meeting the conditions just described, together with others found in practical use of image intensiflers. l

In the above-mentioned patent the screen proposed consisted of a glass plate in which the fluorescent material zinc-sulphide was in a layer submerged just below one face, the photo-electric material cesiated antimony forming a thin coat superposed on the outside of the glass face. The making of such screens is described in detail in my copending applications, Serial Nos. 101,964 and 200,368, filed respectively June 29, 1949, and December 12, 1950. However, while highly sensitive and satisfactory for many uses, such screens have been found to have one property which is undesirable for certain other fields of use. That property is a tendency of the zinc-sulphide to store up a rather minute fraction of the X-ray energy incident upon it, and then to emit it, as radiation to which the photo-electric material is responsive, slowly for hours after the exciting X- rays have been cut off. The resulting continued emission of electrons for hours from the photoelectric surface produces a background glow on the electron-phosphor output screen which is not serious during a `half-hours or so use of the X- ray equipment, but after several hours use the "background glow becomes bright enough to make it diicult to see ner shades and contrasts in the` observed picture. A 24 hour rest-period in a dark chamber is necessary to discharge the stored energy in the iluorescent material suiciently to restore the uorescent screen to its initial condition.

I have found that this difficulty of background glow can be overcome by employing for the fluorescent screen zinc-cadmium-sulphide in place of the zinc-sulphide. However, construction of a zinc-cadmium-sulphide layer separated from a photo-emissive layer by a thin partition of glass capable of operating in the high-vacuum conditions employed in image intensifier tubes poses new problems and difliculties of great severity.

One object of my invention is accordingly to produce a screen having a uorescent layer and a photo-emissive layer separated by a very thin layer of glass, which screen shall be capable of long-continued use without undesirable development of background glow.

Another object of my invention is to provide a screen in which a thin wall of glass separates a photo-emissive layer from a fluorescent material of a type which ceases to emit substantial radiation within a period less than a minute after exciting radiation is cut ofi".

Still another object is to provide a screen having a diameter of the order of 6 inches in which a layer of glass of a thickness between 0.004 inch and 0.010 inch supports on one side a layer of zinc-cadmium-sulphide and on the other side a photo-emissive layer.

Yet another object is to provide a screen in which a glass layer as described in the preceding paragraph supports on one side a layer of zinccadmium-sulphide and on the other side a coating of cesiated antimony.

Another object is to provide an image intensier in which a pattern of exciting radiation falls on a layer of fluorescent material which reradiates it without substantial long-term energy storage to produce an electron-image on a photo-emissive stratium. so closely spaced to said material that the electron-image is an accurate replica of said pattern.

Yet another object is to provide a uorescent screen in which the fluorescent material is supported with a minimum of matter capable of scattering exciting radiation or emitted radiation present in the respective paths traversed by said radiations within the confines of the screen.

A further object is to provide means for causing an incoming pattern of exciting radiation to generate an outgoing pattern of light with the maximum possible preservation of contrasts bctween adjacent areas of the patterns.

Yet another Objectis to provide a screen for transforming exciting radiation to electron emission in which a cesiated antimony photo-electric layer is excited by light from a zinc-cadmium sulphide in which the percentage of cadmium sulphide present is greater than that which would yield light of the color at which the cesiated antimony is photo-electrically most efcient.

Other objects of my invention will become apparent upon reading the following description taken in connection with the drawings, in whichl Fig 1 shows an X-ray image intensifier apparatus of the type in which my invention is to be used; and

Fig. 2 is a sectional View of a fluorescent screen constructed in accordance with my invention.

Referring in detail tothe drawings, Fig. 1 shows schematically an X-ray image intensier in which a conventional X-ray tube l sends a beam through an object 2 being observed into incidence with the input screen 3 of an image intensier tube 4 of the general type described in the above-mentioned Mason and Coltman patent. For present purposes it is considered sufiicient to say that screen 3 is of glass having a layer 5 of zinc-cadmium-sulphide or the like on its convex face, and a layer 5 of some photo-emissive material such as cesiated antimony on its concave face. The light-pattern generated in input layer 5 by the X-rays which have passed through the object 2 strikes the thin photo-emissive layer 6 and causes its free surface to emit an electron-image which is accelerated by suitable electric elds within tube Il into incidence on an electron-phosphor output screen l' at the opposite end of tube l where it produces a light-image which is the greatly intensified copy, to reduced scale, of the light-image generated in input screen 3. An observer views the light-image on output screen 'l through a magnifying optical system 8.

The input screen 3, shown in more detail in Fig. 2, may be of glass, shaped like a watch crystal and having a diameter of about 6 inches and a radius of curvature of about 10% inches. Its thickness is between .004 and .010 inch but preierably between .005 and .008 inch. On its concave side is a very thing layer 9 ofY substantially transparent electrically conducting material believed to be a vitreous form of tin oxide. On its convex side is a layer 5` of an X-ray fluorescent inorganic powder frequently also called a phosphor held by a glassy inorganic binder. The glass support 3- is a clear commercial soda lime glass. Other glasses could be used but at present are not commercially available in thin enough sheets. 1f the diameter of the screen 3 were reduced, then thinner sheets of Cglass could be used. While a screen six inches in diameter and .004 to .005 inch thick has been made, it is considered too fragile for commercial production.

Any of the following ve commercially available phosphor powders may be used in forming the layer 5; zinc-cadmium-sulphide, zinc sulphide, cadmium tungstate, calcium tungstate, andA barium lead sulfate. The rst one is preferred, one commercial form ofwhich is known as Patterson B phosphor. While the above list represents the best X-ray phosphors now available, it is believed any inorganic phosphor which can be heated in vacuum at 400 C. without great deterioration can, however, be used to form the iiuorescent layer of the screen. Such phosphors would include, for example, zinc sulphide selenides, zinc-cadmium-sulphide selenides, zinc silicates, zinc oxide, and alkali earth phosphates. It is desirable that the average size of the phosphor powder particles be greater than ten microns and preferably about or greater than twenty microns.

The binder` which is used to attach the phosphor to the glass is preferably a water solution of potassium silicate. lThe solution used consists of 3 to 15 per cent but preferably about 15 per cent by weight of potassium silicate in water and the potassium silicate has a ratio of silica to potassium oxide of'about ve to two. Other silicate solutions such as sodium silicate, lithium silicate or ethyl silicate may be used.

One method of preparing a screen 3 is as follows. Having obtained a thin watch glass ofthe properl size and shape, it is cleaned, placed on a support having the same shape on top as the glass and heated to about 600 C. in an oven. Upon removal from the oven it is sprayed with a solution of stannic chloride, such as a solution about equal proportions of anhydrous stannic chloride, methyl alcohol and glacial acetic acid. The exact nature of the stannic chloric solution is not considered a part of this invention since many diiferent solutions are known to be useful. The solution leaves a thin layer 9 of substantially transparent, conducting tin oxide on the glass. High conductivity is not required for this layer and any resistance less than one megohm per square is considered satisfactory.

After the glass has cooled, it is placed convex side up in a vessel containing a solution of about 15 per cent potassium silicate to a depth of several inches. About mg. of phosphor powder of each square centimeter to be covered is then sprinkled more or less uniformly through the solution, or the phosphor powder is mixed with some additional solution and poured in as a suspensionand allowed to settle through the solution covering the watch glass. The solution in which the glass was placed helps to disperse the powder and results in a more uniform deposit. If desired, a cylinder of ve and a half inches diamter may be placed on the watch glass to define the area where the phosphor powder will deposit and thus leave a narrow margin around the outside of thesix inch glass disc 3 free of phosphor deposit 5. A solution depth about equal to the diameter of the screen to be covered is considered adequate but is not critical and would be chosen in accordance'with the uniformity desired and method of depositing the phosphor powder. For some powders such' as znc-cadmium-sulphide and cadmium tungstate it isdeisrable to add a surface active or wetting agent tol the solution to prevent the phosphor powder from floating on the surface if it issprinkled in dry form. After the phosphor is settled the solution is removed by siphoning or draining.

The glass 3 with the phosphor is held horizontally while it is removed from the vessel and the potassium silicate solution is washed off from the concave side with a jet of distilled water. The wetv phosphor is then slowly dried and baked up to about 400 C'. to drive out moisture from the silicate binder. Ifn desired, thev phosphor layer may be wetted with additional potassium silicate solution-and-dried to provide better chemical protection against deleterious conditions to which the screen may be exposed. One such condition is the presence of cesium vapor when a oesiumantimony photo-emissive layer'l is applied to the concave side of the screen. A thin aluminum layer may be applied on the phosphor1 layer, either in the form of thin aluminum foil or by evaporation to prevent phosphor particles falling off into the tube.

The procedure outlined above may be varied in many Ways Without departing from the basic structure of a thin glass sheet .004 to .010 inch thick with an inorganic phosphor layer on one side and a transparent conducting layer on the other. For example, the conducting layer which is believed tc be predominantly tin oxide may contain small quantities of other materials and may be applied by dipping and washing instead of spraying with so-lutions or with vapors of tin compounds other than stannic chloride. The phosphor layer may be applied by settling, painting or flowing, The preferred weight of phosphor is about 100 milligrams per square centimeter but this depends on the light output and resolution desired and weights of 50 to 150 ing/cm.2 are considered suitable for image intensier tube use. However, these gures do not represent limits within which the screen can be made since obviously one could equally Well apply any amount less than about one gram per cm.2 by the same methods. If the screen is to be used inside a vacuum tube the binder must be an inorganic material. After the potassium silicate binder is dried by baking, its dry weight is about 2 to 5 per cent and preferably about 3 per cent of the weight of the phosphor powder. This is sufficient to hold it firmly and protect it from attack by cesium vapor during the processing of the cesium-antimony photosurface. These Weight limits could be increased or reduced by a factor of two without interfering greatly with the operation of the screen in the X-ray image tube.

While potassium silicate and sodium silicate solutions are convenient to use as binders of the phosphor particles 5 to the glass surface very low melting glasses might be substituted for them.

While I have described the diiculty eliminated by the use of Zinc-cadmium-sulphide phosphor as arising from the eiect of a minute residual energy accumulated during hours of operation, there are certain situations in medical X-ray practice where very rapid movements are desired to be observed; e. g., some actions of the human heart. The rapid decrement of stored energy radiation characteristic of zinc-cadmium-sulphide and the like are of advantage in even short-time use of the image intensier in such cases.

It appears that the extremely small length of the path of both exciting and emitted radiations Within the confines of the glass avoids scattering and halation which blur the clarity and contrast in screens in which the paths through the glass are not minimized.

It is also found that zinc-cadmium-sulphide is a more efficient absorber of X-rays than zincsulphide of the same thickness and that this more than offsets its lesser efliciency in converting absorbed energy into radiated light.

Zinc-cadmiurn-sulphide is a composite in which the ratio of cadmium-sulphide to zinc-sulphide may be varied over a wide range. Increasing the percentage of cadmium-sulphide shifts the color of the fluorescent light produced toward the red end of the spectrum. Cesiated-antimony has a Value of electron-emission which is a maximum toward the blue end of the spectrum; but I find that the maximum efiiciency of X-ray energy to electron-emission is not attained by using zinccadmium-sulphide of proportions which yield light at the blue end of the spectrum, but results from using a percentage of cadmiumsulphide which yields a green or green-yellow fluorescence. This is probably due to the increased X-ray absorption which results from adding to the percentage of cadmium sulphide present.

I claim as my invention:

1. A screen comprising a sheet of soda-lime glass from 0.005 to 0.008 inch thick having on one side a layer consisting essentially of phosphor powder selected from the group consisting of zinc-cadmium-sulphide, cadmium tungstate, calcium tungstate and barium-lead-sulphate, bound together with a binder selected from the group which consists of sodium silicate, potassium silicate, lithium silicate and ethyl silicate, said sheet having on its other side a transparent layer of material having a resistance not over one megohm per square and coated with a photoelectric layer.

2. A screen comprising a sheet of soda-lime glass from 0.005 to 0.008 inch thick having on one side a layer consisting essentially of zinc-cadmium-sulphide, bound together With a binder selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate, said sheet having on its other side a transparent layer of material having a resistance not over one megohm per square and coated With a photo-electric layer.

8. A screen comprising a sheet of glass from 0.005 to 0.008 inch thick having on one side a layer consisting essentially of Zinc-cadmiumsulphide, bound together with a binder selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate, said sheet having on its other side a transparent layer of material having a resistance not over one magohm per square and coated with a photoelectric layer.

4. A screen comprising a sheet of soda-lime glass in the form of a watch-glass from 0.005 to 0.008 inch thick having on one side a layer consisting essentially of phosphor powder selected from the group consisting of zinc-cadmium-sulphide, cadmium tungstate, calcium tungstate and barium-lead-sulphate, bound together with a binder selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate, said sheet having on its other side a transparent layer of material having a resistance not over one megohm per square and coated with a photo-electric layer.

5. A screen comprising a sheet of glass from 0.005 to 0.008 inch thick having on one side a layer consisting essentially of zinc-cadmium-sulphide, bound together with a binder selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate, said sheet having on its other side a transparent layer of material having a resistance not over one magohm per square and coated `with cesiated antimony.

6. A screen comprising a sheet of glass from 0.005 to 0.008 inch thick having on one side a layer consisting essentially of zinc-cadmium-sulphide, bound together witha binder selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate, said sheet having on its other side a transparent layer of material having a resistance not over one megohm per square and coated with cesiated antirnony, said binder weighing from 2 per cent to 5 per cent of as much as the phosphor.

7. A screen comprising a sheet of glass from 0.005 to 0.008 inch thick having on one side a layer consisting essentially of zinc-cadmium-sulphide in which cadmium sulphide is present in sucient amount to make its uorescent light of a green-yellow color, bound together with a binder selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and 8 ethyl silicate, said sheet having on its other side a transparent layer of material having a resistance vnot over one megohm per square and coated with cesiated antimony.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,289,921 Massa July 14, 1942 2,454,499 Byler'et al Nov. 23, 1948 2,489,127 Forgue Nov. 22, 1949 2,523,132 Mason et al Sept. 19, 1950 2,533,809 Hushley et al Dec. 12, 1950 v2,554,257 Longini et a1 May 22, 1951 2,586,304 Coltman et al Feb. 19, 1952 

1. A SCREEN COMPRISING A SHEET OF SODA-LIME GLASS FROM 0.005 TO 0.008 INCH THICK HAVING ON ONE SIDE A LAYER CONSISTING ESSENTIALLY OF PHOSPHOR POWDER SLECTED FROM THE GROUP CONSISTING OF ZINC-CADMIUM-SULPHIDE, CADMIUM TUNGSTATE, CALCIUM TUNGSTATE AND BARIUM-LEAD-SULPHATE, BOUND TOGETHER WITH A BINDER SELECTED FROM THE GROUP WHICH CONSISTS OF SODIUM SILICATE, POTASSIUM SILICATE, LITHIUM SILICATE AND ETHYL SILICATE, SAID SHEET HAVING ON ITS OTHER SIDE A TRANSPARENT LAYER OF MATERIAL HAVING A RESISTANCE NOT OVER ONE MEGOHM PER SQUARE AND COATED WITH A PHOTOELECTRIC LAYER. 